1. Field of the Invention
The present invention relates to a projection optical system, exposure apparatus, and exposure method and, more particularly, to a high-resolution projection optical system suitable for exposure apparatus used in production of semiconductor devices, liquid-crystal display devices, etc. by photolithography.
2. Related Background Art
The photolithography for production of the semiconductor devices and others is implemented using a projection exposure apparatus for projecting a pattern image of a mask (or a reticle) through a projection optical system onto a wafer (or a glass plate or the like) coated with a photoresist or the like. The resolving power (resolution) required for the projection optical system of the projection exposure apparatus is becoming increasingly higher and higher with increase in integration degree of the semiconductor devices and others.
As a result, in order to satisfy the requirements for the resolving power of the projection optical system, it is necessary to shorten the wavelength λ of illumination light (exposure light) and to increase the image-side numerical aperture NA of the projection optical system. Specifically, the resolution of the projection optical system is expressed by k·λ/NA (where k is the process coefficient). The image-side numerical aperture NA is represented by n·sin θ, where n is a refractive index of a medium (normally, gas such as air) between the projection optical system and the photosensitive substrate (a wafer or the like) and θ a maximum angle of incidence to the photosensitive substrate.
In this case, if the maximum incidence angle θ is increased in order to increase the image-side numerical aperture, it will result in increasing the incidence angle to the photosensitive substrate and the emergence angle from the projection optical system, and therefore increasing reflection loss on optical surfaces. Thus it will fail to secure a large effective image-side numerical aperture. For this reason, as described later, there is the known technology of increasing the image-side numerical aperture by filling a medium like a liquid with a high refractive index in the optical path between the projection optical system and the photosensitive substrate.
On the other hand, as to the degree of difficulty of production of optical systems, the dioptric projection optical systems comprised of only refracting members have the difficulty degree lower than the catadioptric projection optical systems including a reflecting member, and thus the dioptric projection optical systems are more suitable for mass supply of exposure apparatus. In the dioptric projection optical systems, however, a lens unit with a positive refracting power and a lens unit with a negative refracting power need to be located with a space, in order to satisfy the Petzval's condition on the object side (mask side) with respect to the pupil thereof. For realizing the large image-side numerical aperture of not less than 1 as described above, the object-side numerical aperture also increases therewith.
In the dioptric projection optical systems, it will result in failing to well correct for off-axis aberrations such as coma and, in turn, failing to secure a large still exposure region (effective exposure region: effective imaging area) and to avoid an increase in the size of lenses. Then, it is also conceivable to adopt a method of setting the projection magnification (image magnification) of the projection optical system smaller than normal so as to keep the object-side numerical aperture at a low level, thereby well correcting for the off-axis aberrations and avoiding an increase in the size of lenses. However, if the projection magnification of the projection optical system is set smaller than normal, it will result in increasing the mask-side illumination area optically corresponding to a shot-area having a required size, and thus failing to cover this large illumination area by a currently predominating 6-inch reticle.
Nowadays, the exposure apparatus equipped with the projection optical system having the relatively large image-side numerical aperture adopts the step-and-scan system for implementing scanning exposure of a mask pattern in one shot-area on a wafer while moving the mask and wafer relative to the projection optical system. However, if a configuration wherein a medium is filled in the optical path between the projection optical system and the photosensitive substrate is adopted in order to achieve the large image-side numerical aperture, quick scanning exposure is impossible, for example, because of the intervention of the liquid-like medium, and it could cause a drop of throughput.
The present invention has been accomplished in view of the above problems and an object of the invention is to provide an exposure apparatus and exposure method capable of effecting high-resolution projection exposure with high throughput, while securing a large effective image-side numerical aperture through the intervention of a high-refractive-index medium in the optical path between the projection optical system and the photosensitive substrate, using the dioptric projection optical system easy to produce and the mask of the ordinary size.